Process for the esterification of cellulose



CHARLES s. wnnnna A Patented July 12, 1 932 l ma YORK V m. Dra ing.

This invention relates to at-process for esterifying cellulose and particularlyto a new type of catalyst which isused for theesterification.

The esterfication of cellulose by an organic acid'anhydride in a suitable solvent solution is facilitated by the use of a catalyst. The catalysts which have heretofore been employed are legion including among their number mineral acids, organic acids'andsalts of both inorganic and organic acids which ionize to give a suflicient degree of 'acidity. From a commercial standpoint, however, the catalyst which is most useful is sulfuric acid, due to its'availability andto the facility with which it may be used. The use of this catalyst does not require, nevertheless, critical temperature control and, because of its energetic catalytic action which may result in serious degradation of the cellulose product, necessitates the use of expensive refrigeratingequipment to control the actionwithin proper limits. It is due to these facts that the investigators in this field have endeavored" to replace the sul- 'fi furic acid but, up to the with little success. V t

An object of the presentinvention is to pro vide a process for'the' acetylation'of cellulose in which a new type of'catalystis em ployed. Another object of the inventionis to provide a process which'issafe, rapid, and easily controlled and which willyield cellulose acetate free from serious degradation. Other objects will hereinafter appear.

We have found that cellulosicmaterials, such, for example, as cotton lintersor other similar cellulosic'mate'rials may be readily and easily .acetylated if these'materials be esterified in a bath containing sulfur dioxide and an oxide of nitrogen. The oxides of nitrogen which may be used for this purpose presenttime, mostly include nitrogen peroxide or dioxide, nitrogen tetroxide, or, for that matte'r, any of the higher oxides ofnitrogen or nitric oxide in the presence of air. The use of'our new catalyst results in smooth and uniform acetylation of the cellulose with'litt-le or nodegradation and a product of exceptional It is known that the presence water'with the end of this unsuccessful NEW YORK, A CORPORATION OF NEW rmCE ssron Tire EsruRIrrcArIoN or cELLuLosn v 7 i Applicatioufiled Septeii terl 26, 1929. scram. 395,454.

sulfur dioxide and nitrogen dioxide pro- 0 7 even. msT UD, or Rwmmamjw Yomr, Assrenons To EASTMAN KODAK co r Ny, or nocnns'rnn,

duces nitrosyl sulfuric acid, which, in turn,

decomposes to form sulfuric acid and the oxides of nitrogen when'b oth water and oxygen are present. In the acetylation of cellu-. lose, however, by the usual methods, water cannot exist, for "as soonas any is formed 'in' the solution it'reacts instantly with the acetic anhydride-to form acetic acid. It is, 1

therefore, evident that this compound nitrosyl sulfuric acidis not 1 existent-"in the acetylatmg mixture when thesetwo' gases have-been added thereto and their formation does not *explain'theabilityjof these I o5.

gases to catalyze the reaction. "In order to verify this fact, we attempted to acetylate a cellulosic material with both nitric acid and sulfur dioxide in the presence of the usual acetic-anhydride and acetic acid, a

it being a wellknown fact that sulfur dioxide and nitric acid combine to form nitros'yl sul furic acid, but we found that no appreciable acetylation took place even after a period of e hours at an optimum temperature. At

acetylation V period, gaseous oxides of nitrogen were in'- troduced into the acetylation mixture with the result that almost immediately the reac-' tion commenced and withinone hour indicated nearly complete esterfication of the cellulose. The formation,therefore, of nitrosyl sulfuric acid does not explain the-catalytic action of. our gaseous catalyst, sulfur dioxide-nitrogen dioxide. .While 'our at' tempts to explain the catalytic action be tween these two gases have not'been successful, their use, 'nevertheless," results, by the process that will hereinafter he explained,

in a cellulose acetate'which may be 'acetylat- I z ed to thedesired extent. It will be evident to thoseskilled in this art that these gasesmay be addedduringthe acetylating process either in the pretreatment stage, that is, in the" preparation ofthe H cellulosic material prior to the addition thereto of the 'ac'etio-anhydride, or during the acetylationstep, thatis Whenthe anhydride andthe' catalyst are added tolthe pre;

aseous g treated cellulosic material. The catalyst may beadded to'either ofjthese OOPY steps of the acetylation process,'or to the acetylation bath itself if no pretreatment is employed, either by bubbling the gas into the solution or by first absorbing the gas in a suitable solvent, for example, acetic acid and then adding the solvent thus. charged with gas to theesterifying bath. The nitrogen dioxide or any of the equivalent oxides of nitrogen may, if desired, be dissolved in the acetic anhydride prior to its addition to the second'stage of the acetylation process. While we generally prefer to prepare the cellulose acetate in the so-called do e"orm, that is, dissolved in the acetylating solution, it may be prepared in the so-called fibrousform by addition thereto during the pretreatment of l the cellulosic material or at the start of the acetylation, of a sufiieient quantity of a, non-solvent of the cellulose acetate produced, such, for instance, as carbon tetrachloride and benzene, etc. The preparation of the ester in the fibrous form has some advantages as it renders the ester more easily washed and dried and facilitates later solution. c

. If the. oxides of nitrogen are present in the acetylation solution in excess of the quantity of sulfur dioxide, the cellulose acetate produced will be soluble in acetone. This result is very unexpected because it is usually necessary to acetylate cellulose to the fully acetylated form (the triacetatewhich is generally soluble in chloroform) and later to subject it to a hydrolyzing bath whereby its solubility is changed from chloroform to acetone solubility. The, direct acetylation to acetone solubilityby the use of the oxides of nitrogen alone as a catalyst is thoroughly described in copending application Serial No. 378,557, in which we arejoint inventors with Russell H. Van Dyke, Byv utilizing our process, this hydrolysis step may, if desired, be eliminated. If it be desiredv to pro: duce a cellulose. acetate which is chloroformsoluble, it is merely necessary to maintain in the acetylating bath approximately equal molecular proportions of thetwo gases.

The concentration ofthe gases may "vary considerably according 'to the type of product it is desiredto obtain. We have found that a concentration of approximately 5% to of sulfur dioxide and an equivalent amount of the oxide of nitrogen based, upon the weight of the cellulose originally used,

cellulose acetate is to be obtained directly in the form of the acetone-soluble product, a

hig-herpercentage, of the oxide of nitrogen should be used for instance, approximately 2 times the amount of sulfur dioxide. These concentrations are used when thetemper-atune: of -the reacting mixture-is kept below to50; C". andfiatmospheric pressure is employed; If a higher temperature and pres"- sureibe used, the velocity :of. the reactionwill be necessarily increased and it will likewise .be possible to use a lesser proportion of the catalyst than above stated.

We shall now give several methods of carrying out our process, but it shall be distinctly understood that we are not to be limited by the. detailstherin given, except as indicated in the appended claims.

Example 1.- parts of cotton linters may be placed in parts ofgl-acial acetic acid which from 6 to '10 parts of sulfur dioxide have been dissolved. This mixture is allowed to stand at temperature for 16 to ,18 hours. 150 parts of 85% acetic an,- hydri d'e, into which parts of nitrogen di-' oxide has been dissolved, may be, then added. The container is cooled by a water bathto a temperature ap roximately 23F C. and the reacting mass t oroughly agitated. The

temperature of the Water bath is radually increased to about "40 to 50.? Q. tov o tain' the desired rate of reaction.) After 8' hours a product is formed which'is soluble in chloroform and eth .lene chloride. This ester may be hydrolyze by any of the, approved methods.

Emample 2.:5G pa'rt'sof cotton h'nters may be placedin a,v mixture; of 100 parts of glacial acetic acid, 10, parts of sulfur dioxide and 25 0 parts of: Carbon. tetrachloride. After thorough. mixing thcv mass; is allowed tostand at room temperature for one hour whena mixture of 19D parts of carbon tetrachloride and l70 par ts of acetic anhydride containing 6.8%, of nitrogen dioxide by weight are added with, vigorous, stirring, The considerable heat which is developed is controlled b a, water jacket. and by stirring. Thepro not which; is ofta fibrous nature,

after thorough washingand drying, is found to be soluble in chloroformv and ethylene chloride. This product may likewise be eh-angedto the acetonesoluble cellulose acetate, by any of the approved hydrolyzing procedurese p c I 7 The order of introducing the .gases may be alteredjina-ny mannerfas. to time or succession.v Theacetylation will in no case, however, proceedfuntil both gases. are present.

The quantities ofthe gases; can be varied-considerably, the gas in, the smaller molecular ratio controlling; the, reaction rate. By having one gasv slightly in excess, the par ticular characteristics. imparted to the resulting products by that; gas will be evident. For example, if; sulfil'r dioxide isin molecular excess, a decrease in fl efviscosity of the resulting product is obtained. withv a substantial bleaching ofthe: cellulose acetate, while if nitrogen dioxidebe in excess, an increased viscosity results. with a, direct production of acetone soluble celluloseester if sufficient excess be added.

It is evident from a study of the above de- K seribedzprocessthattheuproportionsofithese gases may be varied through wide limits and various oxides of nitrogen may be employed with the production of a cellulose acetate either in solution or in the fibrous form of the chloroform or acetone soluble variety without in any way departing from this invention or sacrificing any of the advantages derived therefrom.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. The process of making cellulose acetate which comprises treating cellulose with an acetylating bath and catalyzing the reaction with sulfur dioxide and an oxide of nitrogen.

2. The process of making cellulose acetate,

which comprises treating cellulose with a non-solvent acetylating bath and catalyzing the reaction with sulfur dioxide and an oxide of nitrogen.

3. The process of making cellulose acetatev which comprises treating cellulose with an acetylating bath and catalyzing the reaction with sulfur dioxide and nitrogen dioxide.

4. The process of making cellulose acetate which comprises treating cellulose with an acetylating bath containing carbon tetrachloride and a catalyst consisting of sulfur dioxide and nitrogen dioxide.

5. The process of making cellulose acetate which comprises treating cellulose with an acetylating bath containing sulfur dioxide and an amount of nitrogen dioxide in excess of that of the sulfur dioxide.

6. The process of making cellulose acetate which comprises treating cellulose with a non-solvent acetylating bath containing sulfur dioxide and an amount of oxide of nitrogen in excess of that of the sulfur dioxide.

7. The process of making cellulose acetate which comprises treating cellulose, with a solution of acetic acid containing dissolved sulfurdioxide subsequently adding thereto dride solution containing nitrogen dioxide and continuing the reaction until an acetic anhy the desired cellulose acetate is obtained.

8. The process of making cellulose acetate which comprises treating cellulose with an acetic acid solution containing carbon tetrachloride and dissolved sulfur dioxide, subsequently adding acetic anhydride in which there has been dissolved nitrogen dioxide in an amount greater than the sulfur dioxide which was added to the bath with the acetic acid, and continuing the reaction until an acetone-soluble cellulose acetate is obtained.

Signed at Rochester, New York, this 21st day of September,1929.

' CHARLES S. WEBBER.

CYRIL J. STAUD. 3

GERTIFIGATE OF CGRREGTWN.

Patent No. 1,866,686. July 12, 1932.

GHARLES S. WEBBER ET AL.

the printed specification of the follows: Page 1, line 17, strike tent should be read'with this eeerti oi the ease in the tied that error appears in quiring correction as that the said Letters Pa he same may content: to the r It is hereby eerti above numbered patent re out the word "not-, and correction therein that t Patent Office.

Signed and sealed this 1st day st November, A. D. 1932.

M. J. Moore, Acting Commissioner oi Patents.

(Seal) 

